The simplest way of varying the supply voltage to an electric motor, or resistive/inductive load, is that of using a Triac device effective to partialize the main voltage being applied to the motor. This technique is cost-efficient, but has a problem in that it introduces harmonics in the current waveform of the main supply.
Stringent European standards have set restrictions on the harmonic contents of main current, and today's trend favors the use of circuit devices based on bridge structures which produce PWM (Pulse Width Modulation) signals and are less prone to introduce harmonics.
Several types of drive circuits for a.c. motors are known from the literature. These circuit types use basically d.c./a.c. switches with inverters, whereby a sinusoidal voltage is output which can be varied in amplitude and frequency in quite an independent manner. All these conventional type circuits are based on bridge circuits producing PWM signals.
The d.c. voltage is obtained by rectifying and filtering the main supply voltage according to a scheme which is known per se and essentially as shown in FIG. 1 of the accompanying drawings.
A rectifier diode is connected in parallel to a filter capacitor, and to a switch having the electric motor, or another electric load such as a lamp or an electric oven, connected across it. These drive circuits have a drawback in that, additionally to introducing harmonics in the waveform of the main current, they incorporate fairly expensive components.
On the other hand, approaches based on the use of Triac devices lead to acoustic noise being generated by d.c. motors, when such motors are regulated by phase partializing. Such is the case with kitchen hoods, for example, where this technique is often used for supplying and driving the suction fans for cost reasons.
The higher order harmonics thus produced cause metal parts and the magnetic circuits associated with the motor to vibrate, thereby producing a rattling noise which is objectionable by the house occupants.
For varying the voltage applied to an a.c. load, it has been proposed to drive electric motors by a partialized main voltage sinusoid, as achieved by controlling the conduction angle of a thyristor. In this way, the voltage at the load is made to depend on the turn-on angle alpha of the thyristor. However, not even this method can solve the problem of harmonics appearing on the main current.